Current treatment options for patients with late stage or metastatic cancer are limited and, in many cases, ineffective. Although combinations of surgery, radiation therapy, and various chemotherapy regimens can be effective, disease relapse remains a major complication. Induction of anti-tumor immunity is an emerging treatment for many types of cancers, especially those where chemotherapy is modestly effective. Although promising, for several reasons, it is predicted that anti-tumor immunocompetent cells alone will not completely eradicate tumor growth in humans. In addition, combining this treatment with chemotherapy is problematic because cytotoxic chemotherapy regimens can severely affect cell-based therapies. To overcome these limitations, we developed several gene-transfer-based methods for conferring drug resistance to cytotoxic immunocompetent cells. Using a model gene therapy system, we showed that significant improvement in animal survival is achieved by combining hematopoietic drug- resistance gene therapy and in vivo expansion of HSC-derived immunocompetent cells, where 100% of mice inoculated with a sarcoma tumor cell line achieve long-term cure. We propose to further develop this novel use of gene-modified HSCs by testing the hypothesis that protection of immunocompetent cells from the toxic effects of chemotherapy will allow for the combined use of chemotherapy and immune modulating agents, and the combined use will be more effective than the single agents. The ultimate goal of this proposal, therefore, is to 1) develop a detailed understanding of the consequences of genetically engineering immunocompetent cells, and 2) expand the usefulness of this gene therapy treatment modality to a clinically relevant animal model. Public Health Relevance: Neuroblastoma is a childhood disease with very poor prognosis. The goal of this proposal is to develop novel strategies that combine drug-resistance gene therapy and immunotherapy. If successful, it may also be possible to generalize this new treatment strategy to other cancers.